In the PET inspection used for the diagnosis of diseases such as cancer, 18F-labeled compounds are used as a probe. The 18F-labeled compounds are produced, for example, by the following liquid phase synthesis method, solid phase synthesis method, and the like.
Liquid phase synthesis method: A precursor compound to be labeled (a compound to be labeled) is allowed to react with a very small amount of 18F ions. As a result, a reaction product contains excess unreacted precursor compound to be labeled, thus requiring a very large amount of effort for the purification of an object compound. Furthermore, in order to overcome the low reactivity of fluoride ions, a highly reactive precursor compound to be labeled, that is, a low-stability precursor compound to be labeled is used in many cases.
Solid phase synthesis method: A precursor compound to be labeled immobilized on a solid phase is used, and only a reaction product is removed in a solution, thereby making the separation of the unreacted product and the reaction product easy (Patent Literature 1, Patent Literature 2, and Non Patent Literature 1). This method is expected to be an effective technique of facilitating the purification of a compound. However, a precursor compound to be labeled immobilized on a solid phase generally has a reduced reactivity. Furthermore, the immobilization on a solid phase reduces volume efficiency and increases the amount of solvent required, thus making it difficult to keep the concentration of fluoride ions at a high level. For this reason, although the purity of the compound obtained is high, it is difficult to obtain a sufficient amount of product. Such a problem of a solid phase synthesis method is pointed out also in Romain Bejot et al., Angew. Chem. Int. Ed. 2009, 48, pp. 586-589 (from the second line from the bottom of the left column to the third line of the right column on page 586).